This invention relates to processing rigid polymers and more particularly to internal and external lubricants used in extruding rigid polyvinyl chloride (PVC).
The necessity of adding a lubricant to rigid PVC as a process control or aid is well recognized yet remains one of the least understood types of polymer additives. In processing high melt viscosity polymers such as polyvinyl chloride (PVC) by extrusion, milling, calendering and injection molding, the shear forces applied cause excessive frictional heat which may lead to serious thermal stability problems. Another problem in processing PVC is to assure that the polymer releases from metal surfaces of the processing equipment. To solve these problems two types of lubricants are used. Lubricants which lower the melt viscosity and control frictional heat build-up are called internal lubricants while substances which promote release are called external lubricants. These materials are used in relatively small amounts since an excess will cause processing and stability problems and structural weakness in the ultimate product. In the processing of polymers such as PVC discrete particles are subjected to stress and heat until there is fusion of the discrete particle and a resulting loss of particle identity. An excess amount of an external lubricant will tend to coat the individual particles and while promoting a slippage between particles, will delay fusion.
The role of the external lubricant is to reduce the surface tackiness of the polymer, particularly during fabrication, thus reducing the tendency of the polymer to adhere to metallic surfaces. The desired degree of lubricity is something less than total release. Consequently, an external lubricant is functionally distinguishable from a mold release agent which is used to coat the metal surface to promote total release. In addition, the denotative definition of lubricant is inapplicable in that an external lubricant of this invention controls surface friction between a melt and a solid surface by incorporation of the lubricant into the melt rather than control of friction by addition of a lubricant film between solid surfaces. The external lubrication that is referred to in this invention can be most easily established by measuring the time required for a lubricated polymer to exhibit sticking to the metal walls of a dynamic mill at elevated temperatures. Characteristically, as the amount of external lubricant added to the polymer is increased, the adherence of the polymer to metal surfaces, such as mill rollers, will decrease. An over externally lubricated polymer would never show stickiness of softness during processing. If the concentration of external lubricant becomes too high, polymer particles will not fuse into a continuous mass in an extruder.
The role of the internal lubricant is to reduce the internal friction within the polymeric melt, which includes reducing heat build-up when the polymer is subjected to stress. Because of the characteristic high melt viscosities of rigid PVC an internal lubricant is usually viewed as being necessary to improve flow properties. Their use will result in an economic advantage in that less work will be expended at a given set of processing conditions. In addition, improved product appearance usually results, particularly with respect to improved surface appearance. An internal lubricant will promote fusion.
The internal lubricants of this invention are distinguishable from what are classified as plasticizer-type additives in several ways. Ideally the improved flow properties associated with an internal lubricant are observed only under fabricating conditions (elevated temperatures and pressures) without influencing the physical properties of the plastic at ambient conditions. In contrast, plasticized PVC is flexible and pliable at room temperature. A true lubricated PVC should not show softness. The internal lubricant is distinguishable pragmatically from a plasticizer in that it is used in relatively low concentration whereas a plasticized PVC contains relatively large quantities of additive. In fact, the internal lubricants cannot be added to PVC in high concentrations because they are fundamentally incompatible with the plastic while plasticizers are compatible. This compatible/incompatible dichotomy is somewhat analogous to solubility/insolubility categorization but more descriptively a matter of degree of solubility. However, a compound functional as a plasticizer at high concentrations will reduce melt viscosity and thus act as an internal lubricant. At low concentrations in rigid PVC, conventional plasticizers cause embrittlement, reduced impact strength and promote flow which can cause distortions in a rigid structure. These conditions are extremely detrimental in uses such as in PVC pipe. A lack of internal lubrication causes heat buildup in processing and results in a rough surface and degraded product.
Although the mechanism of lubrication is not well understood, certain overall principals have evolved historically. The categories of external vs. internal have been viewed historically as mutually exclusive. This mutual exclusivity was reaffirmed by a mechanistic interpretation which viewed the external lubricant as being incompatible to such an extent that it tends to come to the surface during calendering, extrusion or other processing. Whereas this mechanism could not be tolerated for an internal lubricant which must be incompatible at ambient conditions but compatible under processing conditions. It has been observed that if an external lubricant is present in a PVC composition, addition of an internal lubricant will enhance the effectiveness of the external lubricant.
This has led to the commonly expressed belief that what is required for internal lubrication is a 14 to 18 carbon aliphatic hydrocarbon chain terminated in a highly polar end group (the detergent or soap-like theory). Thus, the salts of fatty acids such as stearates, stearyl alcohols and monoglycerides of fatty acids are known internal lubricants. While the salts of linear aliphatic carboxylic acids of about thirty carbon atoms are external lubricants. The rationale here is that increased polarity and decreased chain length result in improved compatibility. Consistent with this are the known external lubricants such as paraffin wax and polyethylene wax. However, the creditability of any simple explanation based on ionic vs. organic moieties of the same molecule is questionable in view of recent observation. The exact opposite rationale can now be justified in view that it is now known that the divalent salts of linear carboxylic acid of about thirty carbon atoms or two linear aliphatic chains of about thirty carbon atoms each interconnected by an ester linkage will exhibit both internal and external lubricant properties. Also, compounds with two long aliphatic chains of 28 to 32 carbon atoms attached to a polar center with ester and soap-ester groups are known to be internal and external lubricants.
Examples of internal lubricants include monoglycerin esters, stearyl alcohol, and stearic acid. Materials that are useful as external lubricants include paraffin oil, straight chain paraffins and polyethylene wax. Dibasic lead stearate is known to insure good release characteristics in compression molding of phonograph records.